Meccano craneMotor reversing switches

In a typical model of a crane, there may be three motors which need to be controlled independently; the motor that rotates the jib, another that raises or lowers the jib, and another that raises or lowers the hook. Each motor can be controlled by a separate switch that allows three positions – forwards, backwards and off. This type of switch is called a Double Pole Double Throw (D.P.D.T.) switch with a centre-off position.

A switch like this will have six connections to it -- three for each pole of the switch – and can be brought as a ‘toggle’ type of switch or a ‘slide’ type. The ideal switch for this application would have a momentary action (it would automatically switch back to the centre-off position after the operator has released the switch), but these are hard to find.

The circuit in figure 1 shows how to wire up one motor to a D.P.D.T. switch.

Figure 1: DPDT switchHow it works

The switch in figure 1 is a Double Pole slide switch in which connections A, B and C form one pole and connections D, E and F form the other. Connections B and E are the ‘common’ connections for their respective poles.
If the positive power supply enters at B, and the switch is set to the topmost position, the left-hand connection to the motor becomes positive, and the motor will run in one direction.
If the switch is then set to the lowest position, the supply is crossed over and the right-hand connection to the motor becomes positive, and the motor runs in the other direction.
In the centre-off position, no power is connected to the motor.

See also: Reversing using two relays

Toggle switches Slide switch

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Relays revealed

The relay (shown in figure 2) is another type of switch in which the lever is operated by an electromagnet. This is useful because a low voltage circuit can energise the coil, which then throws the switch to turn on a high-voltage (even mains) circuit driving high-voltage motors or lamps.
Most relays need 9V or more to energise their coils, and they all have maximum current ratings for their switch contacts which should not be exceeded.

Advantages of relays Figure 2: A typical relay

Disadvantages of relaysFigure 3: Relay circuit symbol

There are several connections to the relay, as shown in figure 3. The ‘common’ connection is the one that will be connected either to ‘NC’or ‘NO’. ‘NO’ means ‘Normally Open’, i.e., it is connected to common when the relay coil is energised. ‘NC’ means ‘Normally Closed’, which means it is not connected to common when the relay coil is energised. Two other connections (usually separated from the rest) are to energise the coil (i.e., turn the relay ‘on’).

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The latching relay circuit

The latching relay circuit performs almost the same job as the 'bi-stable' micro-electronic circuit we will meet in a later issue. See figure 4. If the common of one of the relay poles is connected to +Vs and the NO is connected to the coil, when the relay is switched on a current will flow through the switch to the coil, keeping it energised even when the push switch is released. The only way to turn the relay off is to remove the power supply.

Figure 4: Latching relay circuit diagramUses in models

What's the point of this?
The NO contact of the relay could be connected to a motor. The motor is able to turn indefinitely until some event (maybe triggered by the motor) momentarily interrupts the power supply, thereby turning off the coil ready for the push button to be pushed again.
This system could be used in a model which needs a ‘Push to Operate’ button. A motor and gearing system in the model can be used to depress another switch to cut the power to the relay after the model has been running for a certain amount of time, or until a certain event has occurred.

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Model Study: The Chocolate Vending Machine

My model of a vending machine uses two of the latching relay circuits described above set up so that one chains to the other. The operation is as follows :

Other notes about the model

The switch S10 is a double-pole switch and the two poles are shown ‘ganged’ with a dotted line. This switch simply bypasses the relays to switch on the motors indefinitely so that the machine can be emptied.

Figure 5: Circuit diagram for the Chocolate Vending Machine

Figure 5: The full circuit diagram for the Chocolate Vending Machine.

(To view full size, click on the image)

Front view Left front close-up Inside side view with side door open showing electronics Right front close-up Rear view with rear door open Inside rear view close-up

Various photos of the Vending Machine  (To view a photo full size, click on it)

 

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All about resistors

'All About...' is a column featuring different electronic components, since you will need to know how to use and identify components if you build some of the circuits described in future issues.

In this issue are some facts about resistors:

What they do

A resistor placed in a circuit will resist the passage of electrical current through the circuit and will therefore alter the voltages in the circuit according to Ohm’s Law...

V = I x R

...which relates voltage (V in Volts) to current (I in Amps) and resistance (R in Ohms). The quantity Ohms has it’s own symbol, the Greek letter Omega (W).

How to identify them

The circuit symbol for the resistor is shown below:

Resistor circuit symbol

The value of a resistor (in Ohms) can be found by looking at the three coloured bands printed on it. A fourth band will also be found further apart from the others — this is the tolerance and need not be considered.

All of the colour bands use the system shown below to obtain a number. The first band gives the first digit of the value, the second band gives the second digit of the value, and the third band gives the number of zeros which must be put after the first two digits. Thus, if the first band is BROWN, the second BLACK and the third ORANGE, the value of the resistor is :

1

0

000

=

10,000W

(Brown)

(Black)

(Orange)

 

or 10KW

Resistor colour bands

This table shows the relation between the colours used on resistors and the value a colour represents Colour Value
Black 0
Brown 1
Red 2
Orange 3
Yellow 4
Green 5
Blue 6
Violet 7
Gray 8
White 9

See also: Identifying five-band resistors.  Five-band resistors

Need some help?!? Why not try the Resistor Colour Code Wizard?
Only works on Microsoft Internet Explorer 4.0 or above.

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Title: Practical Matters -- Soldering

Learning how to solder takes a lot of practice — try making joints onto a stripboard track, and joining two pieces of wire using an old piece of stripboard and small lengths of wire.

Here are some guidelines about soldering:

Soldering tools

Soldering technique

Figure 6: Two wires soldered togetherWhen you first use your soldering iron (or fit a new tip to it), ‘tin’ the tip with solder as the iron heats up. When soldering two wires together or attaching a wire to a component, a process called ‘tinning’ is used, as shown in figure 6. Place the tip of the soldering iron on the wire and rest the end of the solder on the wire (not the iron) until it melts onto the wire. Remove the solder and the iron before too much solder melts onto the wire. Do the same for the other wire or the component lead. Touch the two wires together and run the iron quickly over them. The solder on the wires should flow cleanly and join the wires together.
If the joint breaks when you try to pull the wires apart, either the wires were not tinned enough, or you moved the joint before the solder dried.
 

Figure 7: Soldering to stripboardWhen soldering a component lead to a piece of stripboard or to a Printed Circuit Board (PCB), push the lead through the stripboard hole from the component side and allow about 5mm of wire to protrude (see figure 7). Place the tip of the soldering iron at an angle onto both the stripboard track and the component lead and allow both to heat up. Feed the solder in around the track and you should find that it runs all the
way around the hole and the component lead. If this does not happen, the track or the lead are not hot enough. Allow only enough solder to complete the joint — too little and there will be a visible weak spot, too much and there will be a solder ‘blob’ and you risk the solder splashing into other tracks.
When the iron is removed the solder should look shiny and should be in a pyramid shape.
The joint should not be able to move. Trim the component lead down to the top of the solder.

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Title: Shopping ListThe following lists the electrical parts that are discussed in the articles. Prices and order codes given are taken from the current Maplin catalogue, which is the probably best source of electronic components for the hobbyist in the UK.

If you have access to a company account with Rapid Electronics or RS Electronics you may find these companies are cheaper.

Motor reversing switches
  Maplin Order Code Price Page
DPDT toggle switch with centre-off (chrome) FH01B £1.29 308
DPDT toggle switch non-locking centre-off (chrome) FH03D £1.29 308
DPDT Slide switch (black) FH35Q 59p 312
Relays revealed & The latching relay circuit
  Maplin Order Code Price Page
12V 6A DPDT miniature relay FJ42W £2.99 218
6V 6A DPDT miniature relay FJ43W £2.99 218
Low cost push-to-make switch FH59P 59p 314
All about resistors
  Maplin Order Code Price Page
Starter Pack E24 (contains 1330 low wattage resistors from values 1W to 1MW) FA09K £19.99 222
Starter Pack E12 (contains 610 low wattage resistors from 10W to 10MW) FA08J £9.99 221

Resistors can also be brought individually for around 6p each.

Practical Matters: Soldering
  Maplin Order Code Price Page
Soldering Iron Kit SK-6 (contains Antex soldering iron, stand and solder, plus soldering hints & tips) FY69A £19.99 971
Students Tool Kit (contains soldering iron with two bits, soldering aid, stand, solder, flux, solder sucker, Helping Hands bench assistant, four screwdrivers, long-nose pliers, side cutters, and tweezers, all in a smart grey tool case!) BZ60Q £25.99 1018
Standard 22swg solder reel FY70M £10.99 977
Standard solder dispenser GU97F £1.79 977

Maplin charge £2.50 for delivery on orders under £30.00 inc. VAT.
Prices are taken from the September 2000 - August 2001 Maplin catalogue, and include VAT at 17.5%

Contact their order line on 0870 264 6000 or visit one of their shops.
Their customer service line is 0870 264 6002 and they have a website at www.maplin.co.uk where on-line ordering is available.

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www.eleinmec.freeserve.co.uk

Electronics in Meccano September 1998 -- Issue 1

Edited by Tim Surtell
E-mail: timsurtell@eleinmec.freeserve.co.uk


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